1. Field of the Invention
The present invention relates to a multilayer piezoelectric element to be applied to various applications such as a fuel injection valve for vehicles, precision positioning devices for use in optical apparatus, a driving element for preventing vibration, and a piezoelectric actuator for use in inkjet printers.
2. Description of the Related Art
There is a multilayer piezoelectric element, for use in a piezoelectric actuator and the like, having a pair of outer electrodes formed on surfaces of a multilayer ceramic body composed of piezoelectric layers made of piezoelectric materials and inner electrodes of an electrical conductivity which are alternately laminated or stacked. Applying a specified voltage to the inner electrodes in the multilayer piezoelectric element causes the displacement of each piezoelectric layer, and the multilayer piezoelectric element generates and outputs a driving power.
In recent use, a small sized piezoelectric actuator is applied by a higher electric field under a high pressure environment for a long period of time so as to obtain a large amount of displacement in the actuator. When such a piezoelectric actuator made of a multilayer piezoelectric element is used under a high electric field and a high pressure for a long period of time, it causes a separation of the outer electrodes from the multilayer piezoelectric element or cracks therein, and those phenomena cause breaking of the outer electrodes. The occurrence of those cracks or breaking in a part of the outer electrodes halts a voltage supply and the displacement characteristic of the multilayer piezoelectric element varies during its working. In order to avoid the variation of the displacement characteristic, there are related art techniques which have proposed many types of multilayer piezoelectric actuators. For example, Japanese patent laid open publication JP 2002-9356 has proposed a multilayer piezoelectric actuator element having conductive materials which are separated in position from a multilayer body through outer electrodes. This configuration can suppress the deterioration of the performance of the multilayer piezoelectric actuator element even if cracks occur in the multilayer body or the outer electrodes.
Japanese patent laid open publication JP H8-242025 as another related-art technique has proposed a piezoelectric actuator in which piezoelectric element connecting electrodes of a thin plate shape having notches formed in a wide direction thereof are contacted on side surfaces of a piezoelectric element body with a conductive adhesive agent in order to electrically contact those connecting electrodes with inner electrodes of the piezoelectric actuator.
German patent No. DE 19945933 as another related-art technique has disclosed a piezoelectric actuator in which electric layers are connected to contact pins through contact wires, respectively, in order to keep the conductivity of an entire multilayer body of the piezoelectric actuator.
However, the related-art techniques described above have following drawbacks. Because the primary related-art technique JP 2002-9356 has not adequately keep the electrical conductivity of the entire of the conductive materials and has a small current capacity, it is impossible to apply it to the high-speed actuator technical field such as an injector for vehicles.
Although the secondary related-art technique JP H8-242025 has the effect to reduce a stress applied to the notches formed in the piezoelectric element connecting electrodes of a thin plate shape, cracks are generated in the areas other than the notch formation area where the notches are fixed by the conductive adhesive agent, and those areas cannot adequately reduce the stress applied. The presence of the cracks cannot adequately keep the electrical conductivity between the piezoelectric element connecting electrodes and the inner electrodes, and will vary the displacement characteristic of the piezoelectric element during on its driving.
Still further, the related-art technique DE 19945933 as another related-art technique cannot connect wires to all of the electrical layers in its manufacturing, and thereby cannot adequately keep its durability for a long period of time.
Accordingly, there is a strong demand of providing a multilayer piezoelectric element which is capable of suppressing the occurrence of cracks and breaking therein and of keeping its performance such as a displacement characteristic even if it will be used for a long period of time under various strict environments.